The present invention refers to a combustion plant including a circulating fluid bed, which is maintained by the supply of fuel, inert, fine-ground material and oxygen-containing fluidization gas to a vertical furnace shaft, the upper part of which has an outlet to a separator means, where solid particles are separated from the combustion gases and are returned to the lower end of the furnace shaft by way of a lock.
In a combustion plant of above-mentioned type a cyclone separator will form an important part of the cost, and will also require much space. In a big CFB-boiler a single cyclone will often be inefficient due to its size. A plant with an output of 125 MW is thus preferably provided with two cyclones, which may have a diameter of 5 meters and a height of 17 meters. The total space requirement for these cyclones will be about 2.400 m.sup.3. The consumption of electricity for the cyclones will be about 120 kW, and the cost for the units may amount to 9-10 millions SEK. A similar plant for 250 MW would require twice the number of cyclones, and would mean a corresponding increase in space and costs.
In order to maintain the temperature in the furnace at a desired level, it is advantageous to cool the separated particles before they are reintroduced into the furnace.
Big cyclone separators will limit the space for mounting coolers for the bed particles in the shaft between the separators and the lock for re-introducing the bed material into the furnace. Especially when burning coal it will often be necessary to fit extra heat exchange surfaces, for instance superheaters, in the furnace, if this is not extremely high to ensure sufficient heat surfaces. An alternative is to withdraw bed material downstream of the cyclones for cooling in an external heat exchanger, before re-introduction into the furnace. Both superheater and external heat exchanger will cause considerably increased costs.
An aim of the present invention is to propose a cheaper solution of the problems pertaining to the separation of particles from combustion gases, which requires less space than before and makes possible the mounting of adequate cooling surfaces.